This invention relates to apparatus and methods of chemical-mechanical planarization using ultrasonic imaging. More particularly, this invention relates to a chemical-mechanical planarization pad metrology apparatus that transmits an ultrasonic signal onto the surface of a polishing pad to monitor polishing pad properties in real-time.
Fabricating integrated circuit devices is a complex multi-step process that creates structures with various electrical properties to form a connected set of devices. Multiple layers of conducting, semiconducting, dielectric, and insulting materials are deposited on a substrate during integrated circuit device fabrication. As these devices become smaller and more densely packed, more levels of photolithography and additional processing steps are often required.
Often, imperfect substrate fabrication and imperfect integrated circuit layer deposition result in formation of undesirable topography (e.g., recesses, protrusions, scratches, etc.) on the substrate and on one or more of the deposited layers. Because undesirable topography can compromise the integrity of an integrated circuit device (e.g., a topographical recess in a dielectric layer can impose step coverage problems for the deposition of another integrated circuit layer, and undesirable topology can cause depth of focus issues during photolithography), the substrate and each deposited layer of an integrated circuit device are preferably planarized (i.e., made level) before additional layers of integrated circuit material are deposited.
A common technique used to planarize the surface material of an integrated circuit wafer is chemical-mechanical planarization (“CMP”). Known CMP processes are used to remove undesirable topology from layers of integrated circuit material. The rotating polishing pad mechanically polishes (i.e., removes undesirable topography from) the surface material of the integrated circuit wafer. Concurrently, a fluid-based chemical (i.e., a chemical polishing “slurry”) is dispensed onto the surface of the polishing pad to facilitate the removal of undesirable topography. Chemical polishing slurry may react with the integrated circuit material. That is, the slurry chemically weakens surface material of the wafer so that the surface is more easily removed by the mechanical abrasion of the polishing pad. Chemical polishing slurry may also be an inert liquid applied to the polishing pad. The inert liquid facilitates the removal of mechanically-ground integrated circuit material.
As device dimensions continue to scale down, CMP processes become more critical in the process flow. For example, polishing actions should be performed such that scratches or other defects do not appear on the surface of the polished integrated circuit wafer. Furthermore, in order to achieve uniform planarity, a constant polishing rate should be maintained. Thus, polishing pad maintenance plays a significant role in diminishing the drawbacks of the CMP process.
It has been shown that polishing pad properties, such as pad roughness (or texture), pad groove depth (which determines pad wear and pad erosion), pad density, pad thickness, and elastic modulus, influence CMP removal rates and uniform planarity. However, information that relates polishing pad properties to polishing performance is sparse because of inadequate measurement techniques.
Currently, surface topography measurements are obtained using known optical systems, such as a laser scanning microscope. However, there are significant drawbacks with the use of a laser scanning microscope. First, the CMP pad must be cleaned and dried before it can be examined with the microscope, which is a time-consuming and inefficient process. Also, because scanning laser microscopes are cumbersome, the examination process is performed offline (i.e., outside of the CMP tool), which is also a time-consuming and inefficient process. Furthermore, because CMP pads are typically semi-translucent, scanning laser microscopes and other known optical systems have difficulty resolving scratches and polishing pad defects.
In view of the foregoing, it would be desirable to collect polishing pad data and transmit the collected data in real-time to a processor such that process adjustments may be made during a CMP process.
It would also be desirable to maximize wafer throughput (i.e., the number of wafers processed per unit of time) while determining and monitoring polishing pad properties.
It would further be desirable to provide an apparatus for in-situ CMP pad metrology that uses ultrasonic imaging.